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[ rfc791.ORG : Ham Help : Propagation ]

Born in Petoskey, Michigan, a distant relative of Thomas Edison, Claude E. Shannon proved that binary arithmetic could be used to simplify the arrangement of relays in phone switches. He also proved relays could be used to solve boolean algebra problems. This idea of using switches to perform logic operations underlies the operation of the digital computers we use today.
In 1948, Shannon published A Mathematical Theory of Communication, essentially the first work concerned with modern Information Theory. It discussed the most efficient way to encode information for transmission over an analog channel. The Shannon Limit is the theoretical limit of how much information can travel over a channel with a certain amount of noise and a certain bandwidth.
Shannon also enjoyed juggling, unicycling, chess, and invented a chess playing machine, a rocket-powered pogo stick, and a wearable roulette-predicting computer.
When a change in electric current is induced in a conducting material, a magnetic field is produced, and when a magnetic field cuts through a conductor, an electric current is produced. If the current in a conductor is changed quickly enough, say, a hundred thousand times per second or more, then the collapse of the magnetic field will produce an electric field, the collapse of which will cause a magnetic field and so on. Eventually, a magnetic field will be induced which cuts through the receiving antenna, which will induce a current in it which will be detected by the receiver. This phenomenon is known as propagation, and it's how radio waves get around.

Radio waves get from one place to another in a suprisingly large number of ways. The simplest method is called direct wave. At high microwave frequencies like 12GHz, this is the only way. The RF must be able to get from the transmitter antenna to the receiver antenna with no obstructions. You need line-of-sight between the two.

It is possible to get a radio wave to travel around obstructions or around earth by using various means to bend and bounce the waves. Probably the first method to be discovered is a propagation mode called ground-wave. Radio waves travel along the earth and bend, kind of tripping over themselves as they do. Ground waves are most noticable at low frequencies. In the AM broadcast band, for example, ground waves can go a hundred miles or more. They do pretty well on our ham 160 and 80 meter bands too.

One of the most exciting and easy-to work propagation modes for newcomers to the HF bands and to DX (distance radio) in general is skywave. In skywave propagation, radio waves bouce off the ionosphere and return to earth possibly thousands of miles away. This works best in the middle of the HF bands, like on 20-meters (14MHz). There are slightly less-common flavors of this propagation mode like sporadic-E (bouncing off the ionosphere's E layer), and auroral propagation.

Another fun challenge is boucing off of solid heavenly bodies. Many hams are big into boucing radio signals off the moon, and off of meteors. Some hams create their own propagation modes. A non-profit organization called AMSAT puts up their own satellites for amateurs to use freely. Some of these you can work with a simple HT and a rubber duck. But that's not really a mode of propagation strictly speaking, as the satellite demodulates the signal, modulates it on another frequency and amplifies it on its way out.

VHF and UHF offer a propagation mode called tropospheric ducting. If a warm air mass manages to figure out how to sit on top of a cold air mass, the resulting division between the two air masses can channel VHF several hundred miles. The best I've done with this was 110 miles, and that was mostly over water.

So as you can see, there's almost always a way to get a radio signal from here to there, there being the next county over, next door, the other side of the state, the country or even the world. Propagation is a finnicky beast and the only way to learn is to experiment.